With increasing aged deterioration of plants, it is considered that opportunities to maintain and repair a reactor internal structure and equipment will be increased. In an in-core repair work or like, it is necessary to obtain dimensional information such as geometry of an object and geometry of an access route to a target spot (gap). It is important for such dimensional information to not only depend on design drawing information but also depend on as-built information.
Conventional techniques for gap measurement include a three-dimensional measurement method by stereoscopy using two or more images. The three-dimensional measurement method is capable of performing planar shape measurement at a time and is a method which allows efficient implementation of shortest distance measurement necessary as measurement of a gap between structures (see, e.g., Patent Document 1).